1. Field of the Invention
The present invention generally relates to improvements in the design and construction of a coordinate measuring machine (CMM). More particularly, the present invention relates to an improved CMM having a combination of features which result in high performance and low cost.
2. Description of the Prior Art
Coordinate measuring machines (CMMs) are well known in the art. The CMM typically has a probe mounted on a probe shaft, which is movable along a first axis to make measurements on a part. The probe shaft is typically mounted to a carriage which is movable along a second axis orthogonal to the first axis. The second axis is typically movable along a third orthogonal axis, thus allowing the probe to take measurements in all three coordinate axes. The movement of the probe or carriage is accomplished either manually in one type of CMM, where an operator grasps the probe shaft and physically moves it in the desired direction, or automatically in a more sophisticated type of CMM, where it is moved by an automatic controller driving a motor on each axis.
As is known in the art, there are several types of CMM configurations. The fixed-table cantilever-arm CMM provides easy access to the workpiece from three open sides, but its use is restricted to relatively lightweight parts since the table will deflect under the weight of the part. Furthermore, cantilever designs can have a low natural frequency, and large Abbe errors can occur if uncorrected. Various types of moving-ram horizontal-arm CMMs are also used to probe into horizontal recesses in parts. However, these cantilever designs are still susceptible to either large Abbe errors or horizontal beam vibrations.
Moving-bridge CMMs were then designed to overcome the cantilever-type CMM problems. Although moving-bridge CMMs could be built larger than cantilever machines and had higher natural frequencies, the moving-bridge CMM had problems with the outer leg "walking". To alleviate this problem, the ring-bridge CMM, as described in U.S. Pat. No. 4,594,791, was designed. Another CMM design, called the gantry-type CMM, provided an alternative solution to the "walking" leg problem. The gantry-type CMM has a lower platform and two fixed, rigid, upright pillars to provide support for the horizontal beam. Since the uprights are fixed to the base and are sufficiently rigid, there is no walking of any vertical members.
Coordinate measuring machines have been used for years for industrial quality control to inspect the products of computer numerical control (CNC) machine tools and to check feature locations of parts after machining. In fact, CMMs form the cornerstone of the concept of machined part interchangability, since CMMs can measure parts to a higher degree of accuracy than to which the parts were machined. As a result, part manufacturers can use CMMs to help control the quality of their manufacturing processes. Although the CMM was once considered an exotic tool found only in large manufacturing plants, the increased need for accurate measurement of components has become an important concern for small job shops as well.
However, the CMMs currently available are not particularly adapted for use by small machine shops due to their size and cost. The small job shop requires a CMM system which is compact so that the measuring machine takes up no more room than necessary. Furthermore, a large machine also requires more space in the manufacturing facility, where space is also at a premium. As the size of the machine increases, the required length of precision components, such as bearing rails and measuring scales, also increases, thereby adding to the cost of manufacturing. Many smaller CMM designs are simply reduced versions of their large predecessors, and have the same number of parts and manufacturing requirements. This miniaturization of the CMM does not necessarily reduce its cost. Hence, size and cost are important CMM considerations for small machine shops.
On the other hand, speed and accuracy are also major concerns for these CMM users. In order to accomplish the part measurement, the probe and the carriage must be easily movable. Ideally, the probe and carriage and related parts are lightweight. Typical high-performance coordinate measuring machines have high-speed axes which move quickly from point-to-point while making "measurements on the fly". This is accomplished with the use of a device called a "touch trigger probe", which is rigid until it touches the part surface. As the tip of the probe touches the object's surface, it triggers a signal to the CMM controller to measure and record the position of the CMM axes and to quickly decelerate the axial movements. A special probe linkage then allows the probe tip to be subjected to several millimeters of overall travel. This operating mode allows a series of rapid, high-speed measurements to be made. Greater accuracy can be obtained at the price of speed, depending on the design of the particular machine and probe. Accuracy is also dependent upon the use of a low-friction bearing arrangement, which, in turn, is related to the weight of the movable apparatus which is supported on the base.
Typical small machine shop users measure parts having a tolerance as small as five thousandths of an inch. A rule-of-thumb for CMMs is that the CMM accuracy should be approximately 10% of the part tolerance. In order to achieve such accuracy at low costs, the CMM must be stable, rigid, and perform repeatable measurements. Hence, it is critical that non-repeatable distortions of the supporting structures, such as the base and carriages, be avoided. Such distortions directly cause measurement errors due to a loss of precisely repeatable correspondence between the extent of the probe movement and the distance between the points to be measured. Since the carriages are oftentimes powered in their movement along a respective axis by an off-center driving force, the result is a skewing tendency which could introduce non-repeatable distortion forces acting on the carriage. Moreover, conventional mechanical drives for the carriages, such as ball screws or rack-and-pinions, are sufficiently stiff to provide good positioning of the carriage, but require maintenance and lubrication to avoid wear and misalignment. Conventional pneumatic counterbalance mechanisms, used to counteract the weight of the probe shaft, also require maintenance and adjustment to maintain the CMM accuracy. Hence, speed and accuracy cannot be compromised for size and cost.
A need, therefore, exists for an improved coordinate measuring machine having high performance and low cost for the small machine shop.